1. Field of the Invention
The present invention relates to a digital information embedding/extracting apparatus and method, and a medium including a program for executing the method recorded thereon. More particularly, the present invention relates to an apparatus and method for embedding digital data such as copyright information (hereinafter, generically called digital information) in a digital image signal and extracting the digital information from the digital image signal, and a medium including a program for executing the method recorded thereon.
2. Description of the Background Art
In recent years, it has become increasingly popular to provide information via the Internet. In particular, World-Wide Web (WWW) has been frequently used as an information send/receive service system that integrates images, sound, and the like. Once digital information such as images is made public on a network of the Internet, however, it can be easily copied by an indefinite number of users. This raises troubles such as that a third party illegally copies a copyrighted image without permission of the copyright holder and enjoys secondary use. Illegal copying is also a problem for the deployment of business on the Internet using image-base contents. To take measures against illegal copying, it is demanded to establish technology for protecting copyrights of image signals.
As one of the measures against illegal copying, a digital watermark technique is conventionally known, in which digital information is embedded in image data in a form unperceivable to humans. Using this digital watermark technique, if copyrighted digital information is illegally used, the copyright holder can claim the copyright by extracting embedded digital information and verifying the right.
An example of the conventional digital watermark embedding and extracting method is disclosed in Japanese Laid-Open Patent Publication No. 10-191330 (1998-191330) which will be briefly described as follows.
First, the digital watermark embedding method will be described with reference to FIG. 12.
Data of an original image 1101 is picked up every block 1102 of 8xc3x978 pixels based on normal JPEG or MPEG compression. A discrete cosine transform (DCT) operator 1103 a performs DCT operation for the retrieved data so as to transform the data into frequency components. A quantizer 1104 quantizes the data outputted from the DCT operator 1103 by referring to a quantization table 1105. A watermark data embedding unit 1107 embeds watermark data 1106 in the data that is outputted from the quantizer 1104. Specifically, the watermark data that is outputted from the quantizer 1104. Specifically, the watermark data embedding unit 1107 performs processing which is represented by expression (1) below so as to obtain a new frequency component F(i) including the watermark data 1106 embedded therein.
F(i)=f(i)+avg(f(i))xc3x97w(i)xc3x97constantxe2x80x83xe2x80x83(1)
wherein i denotes the element number of a frequency component of the 8xc3x978 pixel block 1002, w(i) denotes the value of each element of the watermark data 1106, and avg(f(i)) denotes the average of the absolutes (absolutes values) of three neighboring points of each element, that is, avg(f(i))=(|f(ixe2x88x921)|+|f(i)|+|f(i+1)|)/3.
The watermark data embedding unit 1107 includes: a partial average calculator 1108 for calculating the average avg(f(i)) of the absolutes (absolute values) of three neighboring points of the data outputted from the quantizer 1104; a multiplier 1110 for multiplying the output of the partial average calculator 1108 by the watermark data 1106 for each element; a multiplier 1111 for multiplying the output of the multiplier 1110 by a constant xcex1 (scaling parameter) 1109 that is used for changing the size of the watermark data 1106 to be embedded for each element; and an adder 1112 for adding the output of the multiplier 1111 to the output of the quantizer 1104 for each element.
An encoder 1113 encodes the frequency component F(i) outputted from the watermark data embedding unit 1107 based on JPEG or MPEG to generate compressed data 1114 including the watermark data embedded therein.
Next, the watermark extracting method will be described with reference to FIG. 13.
Compressed data 1201 is decoded by a decoder 1202. The decoded data is subjected to inverse-quantization by an inverse-quantizer 1203 and then inverse-DCT operation by an inverse-DCT operator 1204 according to normal JPEG or MPEG decoding, to provide reproduced image data 1205.
A watermark data extractor 1206 retrieves each element from the data that is decoded by the decoder 1202, and calculates F(i)/avg(F(i)) so as t extract data that is presumed to be embedded watermark data, wherein F(i) denotes a decoded frequency component of an 8xc3x978 pixel block and avg(F(i)) denotes the partial average of three neighboring points of the data of each 8xc3x978 pixel block outputted from the decoder 1202, that is, avg(F(i))=(|f(ixe2x88x921)|+|f(i)|+|f(i+1)|)/3.
The watermark data extractor 1206 includes: a partial average calculator 1207 for calculating the partial average avg(F(i)) of three neighboring points of the data; and a divider 1208 for dividing the frequency component F(i) outputted from the decoder 1202 by the partial average avg(F(i)) outputted from the partial average calculator 1207.
An adder 1209 accumulates the extracted data for each 8xc3x978 pixel block outputted from the watermark data extractor 1206 to obtain the sum of extracted data of one screen. Once the sum of extracted data of one screen is obtained, an inner product calculator 1210 calculates an inner product between watermark data 1211 expected to be detected and the sum of extracted data, and outputs a statistical similarity 1212.
The statistical similarity 1212 is calculated in the following manner. First, watermark data W(i) is calculated from W(i)=F(i)/avg(F(i)), and then the sum WF(i) of the watermark data W(i) of one screen is calculated for each element i. The statistical similarity C between the watermark data w(i) expected to be detected and the sum WF(i) is calculated from C=WFxc3x97w/(WFDxc3x97wD) using an inner product of vectors, wherein W=(WF(1), WF(2), . . . , WF(n)), w=(w(1), w(2), . . . , w(n)), WFD denotes the absolute of vector WF, and wD denotes the absolute of vector w.
When the statistical similarity 1212 exceeds a predetermined value, it is determined that watermark data equivalent to the watermark data 1211 has been embedded in the compressed data 1201. In this case, if the watermark data 1211 is data indicating xe2x80x9cduplication prohibitedxe2x80x9d, for example, a reproduction machine that incorporates this watermark data extracting scheme can take some measures against duplication of the reproduced image data 1205, for example.
The above conventional technique has the following problems. During the extraction of watermark data, the frequency component F(i) must be divided by the partial average avg(F(i)) for each element of an 8xc3x978 pixel block. This complicates the operation.
Moreover, during the embedding of watermark data, since the constant xcex1 used for changing the size of the watermark data is fixed, embedding depending on each frequency component of the 8xc3x978 pixel block is not possible. Therefore, if the value of the constant xcex1 is increased to improve the rate of detection of watermark data, the image quality is deteriorated.
Therefore, a main object of the present invention is to provide a digital information embedding/extracting apparatus and method where the construction and operation are simpler, the calculation cost is lower, and moreover deterioration in image quality is less observable, compared with conventional ones, by using a frequency component-dependent weight sequence for each element of a transform coefficient during embedding of watermark data and calculating an inner product between a transform coefficients sequence and a pseudo-random number sequence during extraction of watermark data, and a medium including a program for executing the method recorded thereon.
Another object of the present invention is to provide a digital information embedding/extracting apparatus and method capable of removing only desired digital information from compressed data including a plurality of digital information embedded therein, and a medium including a program for executing the method recorded thereon.
The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a digital information embedding apparatus for embedding inherent digital information in a digital image signal, comprising:
a transform coefficient calculation section for decomposing the digital image signal into frequency components and calculating transform coefficients for a plurality of blocks each composed of a predetermined number of pixels;
a mapping section for mapping the inherent digital information with a pseudo-random number sequence;
an information embedding section for selecting a specific transform coefficients sequence from the transform coefficients calculated in the transform coefficient calculation section and embedding the pseudo-random number sequence in the specific transform coefficients sequence based on a predetermined weight sequence corresponding to elements of the transform coefficients, for each of said blocks;
a quantization section for quantizing the embedding-processed transform coefficients; and
an encoding section for encoding the quantized transform coefficients.
A second aspect of the present invention is directed to a digital information embedding apparatus for embedding inherent digital information in a digital image signal, comprising:
a transform coefficient calculation section for decomposing the digital image signal into frequency components and calculating transform coefficients for a plurality of blocks each composed of a predetermined number of pixels;
a quantization section for quantizing the transform coefficients outputted from the transform coefficient calculation section;
a mapping section for mapping the inherent digital information with a pseudo-random number sequence;
an information embedding section for selecting a specific transform coefficients sequence from the quantized transform coefficients and embedding the pseudo-random number sequence in the specific transform coefficients sequence based on a predetermined weight sequence corresponding to elements of the transform coefficients, for each of said blocks; and
an encoding section for encoding the embedding-processed transform coefficients.
The weight sequence used in the above aspects is preferably set so that as a frequency component of a transform coefficient outputted from the transform coefficient calculation section shifts in frequency from low to high, a value of the corresponding element of the weight sequence is greater.
Also preferably, a quantization table used for encoding of the digital image signal is used for the weight sequence.
Alternatively, the weight sequence may be set dynamically depending on a value of a DC component, or an average of specific AC components, of the transform coefficient outputted from the transform coefficient calculation section for each of the blocks.
Preferably, the information embedding section comprises:
an absolute calculator for calculating an absolute (absolute value) of each element of the specific transform coefficients sequence selected from the transform coefficients outputted from the transform coefficient calculation section;
a first multiplier for multiplying the absolute of each element outputted from the absolute calculator by a first set value;
a second multiplier for multiplying each element of the weight sequence by a second set value;
a first adder for adding an output of the second multiplier to an output of the first multiplier for each element;
a third multiplier for multiplying an output of the first adder by the pseudo-random number sequence for each element; and
a second adder for adding an output of the third multiplier to the output of the transform coefficient calculation section for each element.
The transform coefficient calculation section in the above aspects may perform any of discrete cosine transform, Fourier transform, Hadamard transform, discrete wavelet transform, and sub-band division.
As described above, in the first and second aspects, an original image is transformed into frequency components for each block of a predetermined size, and a pseudo-random number sequence associated with inherent digital information is embedded in the resultant transform coefficients sequence using a weight sequence of which each element corresponds to each element of the transform coefficients sequence. Therefore, embedding of inherent digital information can be realized with simple construction and operation. In addition, it is substantially difficult for the third party who is informed of neither the order and length of the transform coefficients sequence for which embedding is made nor the pseudo-random number sequence to be embedded to decipher the inherent digital information. Moreover, since the pseudo-random number sequence associated with the inherent digital information, not the inherent digital information itself, is embedded, quality deterioration of the compressed data due to the embedding is less observable. If a quantization table used for encoding of digital image signals is used for the weight sequence, deterioration in image quality can be suppressed.
A third aspect of the present invention is directed to a digital information extracting apparatus for extracting inherent digital information embedded in a specific transform coefficients sequence among transform coefficients calculated by decomposing a digital image signal into frequency components by a specific apparatus, comprising:
a decoding section for receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
an inverse-quantization section for performing inverse quantization for data outputted from the decoding section;
a correlation value calculation section for calculating an inner product between the specific transform coefficients sequence among data outputted from the inverse-quantization section and each of a plurality of pseudo-random number sequences including a pseudo-random number sequence associated with the inherent digital information embedded by the specific apparatus;
a pseudo-random number sequence determination section for determining a pseudo-random number sequence embedded in the data based on a value of the inner product outputted from the correlation value calculation section; and
an information generation section for generating inherent digital information mapped with the determined pseudo-random number sequence.
A fourth aspect of the present invention is directed to a digital information extracting apparatus for extracting inherent digital information embedded in a specific quantized transform coefficients sequence among transform coefficients calculated by decomposing a digital image signal into frequency components by a specific apparatus, comprising:
a decoding section for receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
a correlation value calculation section for calculating an inner product between the specific quantized transform coefficients sequence among data outputted from the decoding section and each of a plurality of pseudo-random number sequences including a pseudo-random number sequence associated with the inherent digital information embedded by the specific apparatus;
a pseudo-random number sequence determination section for determining a pseudo-random number sequence embedded in the data based on a value of the inner product outputted from the correlation value calculation section; and
an information generation section for generating inherent digital information mapped with the determined pseudo-random number sequence.
As described above, in the third and fourth aspects, compressed data including a pseudo-random number sequence embedded therein is decoded, and a correlation value is obtained by calculating an inner product between the transform coefficients sequence and the pseudo-random number sequence. The correlation value is compared with a predetermined threshold value, to specify the embedded pseudo-random number sequence and thus generate the inherent digital information. Therefore, extraction of the inherent digital information can be realized with simple construction and operation. In addition, it is substantially difficult for the third party who is informed of neither the order and length of the transform coefficients sequence for which embedding is made nor the pseudo-random number sequence to be embedded to decipher the inherent digital information. Moreover, since the pseudo-random number sequence associated with the inherent digital information, not the inherent digital information itself, has been embedded, the rate of detection of watermark data can be improved.
A fifth aspect of the present invention is directed to a digital information embedding apparatus for removing only desired digital information among a plurality of digital information embedded in a specific transform coefficients sequence calculated by decomposing a digital image signal into frequency components by a specific apparatus based on a weight sequence, comprising:
a decoding section for receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
an inverse-quantization section for performing inverse quantization for data outputted from the decoding section;
a mapping section for mapping the desired digital information with a pseudo-random number sequence;
an information embedding section for selecting a specific transform coefficients sequence in which the desired digital information has been embedded from data outputted from the inverse-quantization section, and embedding the pseudo-random number sequence in the specific transform coefficients sequence;
a quantization section for quantizing the embedding-processed transform coefficients; and
an encoding section for encoding the quantized transform coefficients.
A sixth aspect of the present invention is directed to a digital information embedding apparatus for removing only desired digital information among a plurality of digital information embedded in a specific quantized transform coefficients sequence calculated by decomposing a digital image signal into frequency components by a specific apparatus based on a weight sequence, comprising:
a decoding section for receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
a mapping section for mapping the desired digital information with a pseudo-random number sequence;
an information embedding section for selecting a specific quantized transform coefficients sequence in which the desired digital information has been embedded from data outputted from the decoding section, and embedding the pseudo-random number sequence in the specific transform coefficients sequence; and
an encoding section for encoding the embedding-performed transform coefficients.
Preferably, the information embedding section comprises:
an absolute calculator for calculating an absolute of each element of the specific transform coefficients sequence selected from data outputted from the inverse-quantization section;
a first multiplier for multiplying the absolute of each element outputted from the absolute calculator by a first set value;
a second multiplier for multiplying each element of the weight sequence used by the specific apparatus by a second set value;
an adder for adding an output of the second multiplier to an output of the first multiplier for each element;
a third multiplier for multiplying an output of the adder by the pseudo-random number sequence for each element; and
a subtracter for subtracting an output of the third multiplier from the output of the inverse-quantization section.
Alternatively, the information embedding section comprises:
an absolute calculator for calculating an absolute of each element of the specified transform coefficients sequence selected from data outputted from the inverse-quantization section;
a first multiplier for multiplying the absolute of each element outputted from the absolute calculator by a first set value;
a second multiplier for multiplying an output of the first multiplier by the pseudo-random number sequence for each element; and
a subtracter for subtracting an output of the second multiplier from the output of the inverse-quantization section.
As described above, in the fifth and sixth aspects, only a given pseudo-random number sequence is subtracted from a plurality of pseudo-random number sequences embedded. Thus, it is possible to remove only desired digital information from compressed data including a plurality of digital information embedded therein without causing deterioration in image quality. For example, this processing is useful when it is desired to remove unnecessary author information among a plurality of author information embedded in compressed data.
A seventh aspect of the present invention is directed to a digital information embedding method for embedding inherent digital information in a digital image signal, comprising the steps of:
decomposing the digital image signal into frequency components and calculating transform coefficients for a plurality of blocks each composed of a predetermined number of pixels;
mapping the inherent digital information with a pseudo-random number sequence;
selecting a specific transform coefficients sequence from the calculated transform coefficients, and embedding the pseudo-random number sequence in the specific transform coefficients sequence based on a predetermined weight sequence corresponding to elements of the transform coefficients, for each of said blocks;
quantizing the embedding-processed transform coefficients; and
encoding the quantized transform coefficients.
An eighth aspect of the present invention is directed to a digital information embedding method for embedding inherent digital information in a digital image signal, comprising the steps of:
decomposing the digital image signal into frequency components and calculating transform coefficients for a plurality of blocks each composed of a predetermined number of pixels;
quantizing the calculated transform coefficients;
mapping the inherent digital information with a pseudo-random number sequence;
selecting a specific transform coefficients sequence from the quantized transform coefficients, and embedding the pseudo-random number sequence in the specific transform coefficients sequence based on a predetermined weight sequence corresponding to elements of the transform coefficients, for each of said blocks; and
encoding the embedding-processed transform coefficients.
A ninth aspect of the present invention is directed to a digital information extracting method for extracting inherent digital information embedded in a specific transform coefficients sequence among transform coefficients calculated by decomposing a digital image signal into frequency components by a specific apparatus, the method comprising the steps of:
receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
performing inverse quantization for the decoded data;
calculating an inner product between the specific transform coefficients sequence among the inverse-quantized data and each of a plurality of pseudo-random number sequences including a pseudo-random number sequence associated with the inherent digital information embedded by the specific apparatus;
determining a pseudo-random number sequence embedded in the data based on a calculated value of the inner product; and
generating inherent digital information mapped with the determined pseudo-random number sequence.
A tenth aspect of the present invention is directed to a digital information extracting method for extracting inherent digital information embedded in a specific quantized transform coefficients sequence among transform coefficients calculated by decomposing a digital image signal into frequency components by a specific apparatus, the method comprising the steps of:
receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
calculating an inner product between the specific quantized transform coefficients sequence among the decoded data and each of a plurality of pseudo-random number sequences including a pseudo-random number sequence associated with the inherent digital information embedded by the specific apparatus;
determining a pseudo-random number sequence embedded in the data based on a calculated value of the inner product; and
generating inherent digital information mapped with the determined pseudo-random number sequence.
The step of determining a pseudo-random number sequence in the ninth and tenth aspects preferably includes determining a pseudo-random number sequence providing an inner product greater than a predetermined threshold value as the embedded pseudo-random number sequence.
An eleventh aspect of the present invention is directed to a digital information embedding method for removing only desired digital information among a plurality of digital information embedded in a specific transform coefficients sequence calculated by decomposing a digital image signal into frequency components by a specific apparatus, the method comprising the steps of:
receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
performing inverse quantization for the decoded data;
mapping the desired digital information with a pseudo-random number sequence;
selecting a specific transform coefficients sequence in which the desired digital information has been embedded from the inverse-quantized data, and embedding the pseudo-random number sequence in the specific transform coefficients sequence;
quantizing the embedding-processed transform coefficients; and
encoding the quantized transform coefficients.
A twelfth aspect of the present invention is directed to a digital information embedding method for removing only desired digital information among a plurality of digital information embedded in a specific quantized transform coefficients sequence calculated by decomposing a digital image signal into frequency components by a specific apparatus, the method comprising the steps of:
receiving compressed data encoded and outputted by the specific apparatus and decoding the compressed data;
mapping the desired digital information with a pseudo-random number sequence;
selecting a specific quantized transform coefficients sequence in which the desired digital information has been embedded from the decoded data, and embedding the pseudo-random number sequence in the specific transform coefficients sequence; and
encoding the embedding-processed transform coefficients.
Typically, the digital information embedding/extracting methods described in the seventh to twelfth aspects are implemented by a computer including a memory that stores predetermined program data programmed to realize the steps described above and a CPU executing the program data. The predetermined program data programmed to realize the steps described in the seventh to twelfth aspects may be installed on the computer via a recording medium that can write the program data.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.